cmCTest: Explicitly specify '.tmp' extension on in-progress log files

Do not depend on `cmGeneratedFileStream` to have this default.

Source/cmCTest.cxx

@@ -841,6 +841,7 @@ bool cmCTest::OpenOutputFile(const std::string& path, const std::string& name,
     }
   }
   std::string filename = testingDir + "/" + name;
+  stream.SetTempExt("tmp");
   stream.Open(filename);
   if (!stream) {
     cmCTestLog(this, ERROR_MESSAGE,